1. Field of the Invention
The present invention relates to an electric power conversion device and, in particular, to a method of element arrangement in a cooler of an electric power conversion device using semiconductor elements which is installed in railroad rolling stock.
2. Description of the Related Art
An electric power conversion device is generally composed of semiconductor elements and includes inverter circuits which convert a dc power source to an alternating current, converter circuits which convert an ac power source to a direct current, and the like. Inverter circuits include a variable voltage variable frequency (VVVF) inverter circuit which controls the voltage and frequency of an ac output in variable manner. The VVVF inverter circuit is frequently used in an electric power conversion device.
For example, in dc electric rolling stock of a railroad rolling stock system, a VVVF inverter circuit is generally used as an electric power conversion device, and controls an ac induction motor by converting a direct current to a three-phase alternate current of variable voltage variable frequency. In ac electric rolling stock, a single-phase ac power source is temporarily converted to a direct current by a converter circuit, the dc power source is converted to a three-phase alternating current of variable voltage variable frequency by an inverter circuit, and an ac induction motor is driven.
In the case where an instable dc power source is stabilized or dc voltage is converted to an arbitrary value for use, or in the case where it is necessary to output a dc power source electrically insulated from input, a DC-DC converter circuit is used and an electric power conversion device is used also in the converter circuit.
In a conversion circuit part of such an electric power conversion device using the semiconductor elements, heat is generated due to heat generation losses occurring during the switching of the semiconductor elements and during the flow of a current to the semiconductor elements. Consequently, the heat is discharged to outside the electric power conversion device by a cooler, whereby the temperature of the semiconductor elements is kept at not more than a permissible value thereof. A cooler used in an electric power conversion device is basically composed of a heat receiving part on which semiconductor elements are mounted and a heat radiating part which radiates heat to the ambient air. The heat receiving part is placed in a sealed chamber portion of the electric power conversion device, whereas the heat radiating part is placed in an open chamber portion in communication with the ambient air.
Incidentally, in the DC-DC converter described above, there are known methods in which the applied frequency is increased to reduce the size of an insulation transformer, and among others, techniques based on a method by which switching losses are reduced through the use of a resonance circuit (soft switching) are described in Japanese Patent Laid-Open Publication No. 4-368464 and O. Deblecker, A. Moretti, and F. Vallee: “Comparative Analysis of Twozero-Current Switching Isolated DC-DC Converters for Auxiliary Railway Supply,” SPEEDAM2008.
This circuit configuration is shown in FIG. 7. The DC-DC converter shown in FIG. 7 is composed of a dc voltage source 200, a converter 201 which converts the dc power of the dc voltage source 200 to ac power, a transformer 202 which inputs the ac power outputted by the converter 201, a rectifying circuit 203 which converts the ac power outputted by the transformer 202 to dc power, a resonance circuit composed of a resonance switch 204 and a resonance capacitor 205 which are connected in parallel on the dc output side of the rectifying circuit 203, a filter reactor 206 and a filter capacitor 207 which smooth the dc power outputted by the rectifying circuit 203, and a load 208 connected in parallel to the filter capacitor 207.
The DC-DC converter activates the resonance switch 204 in synchronization with the timing of turning-off of the converter 201 and superimposes a resonance current Iz on a secondary current I2, thereby temporarily enabling the secondary current I2 to be reduced to zero and a primary current I1 to be reduced to the level of only an exciting current of the transformer 202. It is possible to substantially reduce turning-off losses of the converter 201 by turning off the converter 201 in synchronization with this timing.
In the DC-DC converter shown in FIG. 7, the primary current I1 and the secondary current I2 are zero while the semiconductor elements Q1 to Q4 constituting the converter 201 are off, but a returning current continues to flow through the diodes constituting the rectifying circuit 203. When the semiconductors Q1, Q4 constituting the converter 201 are turned on from this state, the primary current I1 and the secondary current I2 begin to flow and the magnitude of the secondary current I2 becomes equal to that of a load current Id. At this time, a current of the same magnitude as the secondary current I2 flows through a half number of the diodes constituting the rectifying circuit 203, and the current in the remaining half number of the diodes becomes zero.
FIG. 2 shows an embodiment of a circuit in which the converter portion of the circuit is used as a three-level circuit. A DC-DC converter 13 is composed of a dc voltage source 10, a filter capacitor 11 (FC1) and a filter capacitor 12 (FC2) which are connected in parallel to the dc voltage source 10, a converter 13 which converts the dc power of the filter capacitor 11 and the filter capacitor 12 to ac power, a transformer 14 which inputs the ac power outputted by the converter 13, a rectifying circuit 15 which converts the ac power outputted by the transformer 14 to dc power, a resonance switch 16 (Qz) which is connected in parallel to the dc output side of the rectifying circuit 15, a resonance circuit 21 composed of a resonance capacitor 17, a filter reactor 18 and a filter capacitor 19 which smooth the dc power outputted by the rectifying circuit 15, and a load 20 connected in parallel to the filter capacitor 19.
The DC-DC converter shown in FIG. 2 activates the resonance switch 16 in synchronization with the timing of turning-off of the converter 13 and superimposes a resonance current Iz on a secondary current I2 of the transformer 14, thereby temporarily enabling the secondary current I2 to be reduced to zero and a primary current I1 to be reduced to the level of only an exciting current of the transformer 14. It is possible to substantially reduce turning-off losses of the converter 13 by turning off the converter 13 in synchronization with this timing.
In the DC-DC converter shown in FIG. 2, the primary current I1 and the secondary current I2 are zero while the semiconductor elements Q1 to Q4 constituting the converter 13 are off, but a returning current continues to flow through the diodes constituting the rectifying circuit 15. When the semiconductors Q1 and Q2 or Q3 and Q4 constituting the converter 13 turning-on from this state, the primary current I1 and the secondary current I2 begin to flow and the magnitude of the secondary current I2 becomes equal to a load current Id. At this time, a current of the same magnitude as the secondary current I2 flows through a half number of the diodes constituting the rectifying circuit 15, and the current in the remaining half number of the diodes becomes zero.
In the converter 13 of three-level circuit which acts in this way, the level of heat generation of the semiconductor elements is in decreasing order semiconductor elements Q1, Q4>semiconductor elements Q2, Q3>clamp diodes D5, D6. In the semiconductor elements Q1 to Q4, there are losses caused by the conduction of a load current Id in an on state and switching losses occurring at turning on and turning off. On the other hand, because only currents flowing in the clamp diodes D5, D6 are resultant from commutating actions caused by switching and losses are small, the calorific value is also small compared with the semiconductor elements Q1 to Q4. The reason why there is a difference in calorific value between the semiconductor elements Q1, Q4 and the semiconductor elements Q2, Q3 is that the semiconductor elements Q1, Q4 are turned off with the primary current I1 on the order of an exciting current of the transformer 14, whereas the semiconductor elements Q2, Q3 are turned off with the current zero, with the result that there is a difference, in particular, in the magnitude of switching losses by turning-off, leading to a difference in the calorific value between the them. A more detailed reason will be given in the embodiments which will be described later.
At this time, by arranging the semiconductor elements Q1, Q4 having large heat generation in places where the cooler has large cooling performance as far as possible and where the semiconductor elements Q1, Q4 are not affected by other heat generating elements placed on the same plane of the cooler, the cooling performance can be utilized to a maximum degree, with the result that it becomes possible to reduce the size and weight of the cooler.
For this reason, regarding an electric power conversion device having a three-level converter circuit using semiconductor elements which is installed under the floor of railroad rolling stock, for example, Japanese Patent Laid-Open Publication No. 2003-79162 describes an arrangement method for semiconductor elements and diodes to be cooled on one cooler 110 (a heat receiving potion) in an electric power conversion device.
FIG. 8 is a circuit diagram showing an example of one phase portion of a converter circuit 2 described in Japanese Patent Laid-Open Publication No. 2003-79162. The converter circuit is composed of an a-system and a b-system of a 2-system parallel configuration. First, the a-system is configured in such a manner that between a dc positive terminal P and a dc negative terminal N, four semiconductor elements composed of a first semiconductor element Q101a to a fourth semiconductor element Q104a are connected in series and a first diode Dd101a and a second diode Dd102a are connected in series. Similarly, also the b-system is configured in such a manner that between the dc positive terminal P and the dc negative terminal N, four semiconductor elements composed of a first semiconductor element Q101b to a fourth semiconductor element Q104b are connected in series and a first diode Dd101b and a second diode Dd102b are connected in series.
Capacitors CF1, CF2 are connected to the series circuits of the first diodes Dd101a, Dd101b and the second diodes Dd102a, Dd102b, and snubber circuits 5 are provided. An ac terminal M is connected to a connection point of the second semiconductor element Q102 and the third semiconductor element Q103, and a neutral terminal C is connected to a connection point of a first diode Dd101 and a second diode Dd102. A filter capacitor FC is connected to between the dc positive terminal P and the dc negative terminal N.
FIG. 9 shows an embodiment of the element arrangement of Japanese Patent Laid-Open Publication No. 2003-79162. The order of magnitude of heat generation losses of each element shown in FIG. 9 is in decreasing order semiconductor elements Q102a (Q102b), Q103a (Q103b)>semiconductor elements Q101a (Q101b), Q104a (Q104b)>clamp diodes Dd101a (Dd101b), Dd102a (Dd102b). As shown in FIG. 9, on the heat receiving part of the cooler 110, the second and third semiconductor elements Q102a (Q102b), Q103a (Q103b) having large heat generation losses are arranged on the windward side of the cooling wind, as well as the first and second diodes Dd101a (Dd101b), Dd102a (Dd102b) and the first and fourth semiconductor elements Q101a (Q101b), Q104a (Q104b), which have small heat generation losses, are arranged on the leeward side of the cooling wind for Q102a (Q102b) and Q103a (Q103b). With this arrangement, it is possible to increase the efficiency of the cooling performance, as a result of which, this arrangement advantageously reduces the size of the cooler 110.
However, this arrangement increases the lateral dimension of the cooler 110 (i.e., the dimension in the direction orthogonal to the cooling wind), and in the case where an electric power conversion device is installed under the floor of railroad rolling stock, the dimension of the cooler 110 in the rail direction increases and a large installation space may sometimes be required. There is the configuration shown in FIG. 10 as an example in which in order to reduce the lateral dimension of the cooler 110, the components are arranged in a multi-stage configuration. Semiconductor elements Q101a to Q104a, Q101b to Q104b having large heat generation are arranged on the windward side, whereas clamp diodes Dd101a (Dd101b), Dd102a (Dd102b) having small heat generation are arranged on the leeward side.
However, the invention of Japanese Patent Laid-Open Publication No. 2003-79162 above has a problem described below. The problem is described below with the aid of FIG. 11. FIG. 11 shows the current paths indicated by PATH II of FIG. 3. It is apparent the current paths assume a large loop that increases the path length. For this reason, the problem with the invention is that the parasitic inductance of the circuit becomes large and that the overvoltage occurring in the parasitic inductance at the turning-off of the semiconductor elements is applied to the elements, causing a damage to the semiconductor elements.
For this problem, in Japanese Patent Laid-Open Publication No. 2003-79162, a measure which involves insertion of the snubber circuits 5 is taken to suppress a rise in the voltage of the semiconductor elements. However, this measure results in an increase in the size and weight of the device due to an increased number of parts and a decrease in the reliability of the device due to an increased risk of troubles in the parts.
The present invention is intended for solving the above-described problem and the object of the present invention is, in an electric power conversion device in which the cooling performance of a cooler is improved by making the most of a difference in heat generation from semiconductor elements, to improve the reliability of the electric power conversion device by suppressing the overvoltage applied to the elements without the need to add snubber circuits by reducing parasitic inductance.